1. Field of the Invention
The present invention relates to an objective lens system for a device for optically recording and reading digitalized information on an optical disk which is utilized as a document file or the like.
2. Description of the Prior Art
In case of recording the information on the disk, a more intensive light energy is required than the case of reading the information. Therefore, not only the numerical aperture, NA on the disk side but also that the numerical aperture NA' on the light source side should be increased in comparison with the case of an objective lens for exclusive use of reading the information. An increase of the magnification, .beta.(=NA'/NA) of the objective lens system may suit the above purpose. As magnification becomes greater, however, an angle of incidence becomes so greater as to cause both spherical aberration and coma which are difficult to be corrected well. An attempt to correct such aberrations with a combination of only spherical lens elements would necessitate a greater number of lens elements forming the objective lens system with its size and weight inevitably increased.
Japanese patent applications laid open Nos. 58-211729, 57-154213 and 59-157607 are cited to show prior art objective lens systems in this field of art.